.TH std::call_once 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::call_once \- std::call_once

.SH Synopsis
   Defined in header <mutex>
   template< class Callable, class... Args >                              \fI(since C++11)\fP
   void call_once( std::once_flag& flag, Callable&& f, Args&&... args );

   Executes the Callable object f exactly once, even if called concurrently from
   several threads.

.SH In detail:

     * If, by the time std::call_once is called, flag indicates that f was already
       called, std::call_once returns right away (such a call to std::call_once is
       known as passive).
     * Otherwise, std::call_once calls INVOKE(std::forward<Callable>(f),
       std::forward<Args>(args)...). Unlike the std::thread constructor or std::async,
       the arguments are not moved or copied because they do not need to be transferred
       to another thread of execution (such a call to std::call_once is known as
       active).

     * If that invocation throws an exception, it is propagated to the caller of
       std::call_once, and flag is not flipped so that another call will be attempted
       (such a call to std::call_once is known as exceptional ).
     * If that invocation returns normally (such a call to std::call_once is known as
       returning), flag is flipped, and all other calls to std::call_once with the same
       flag are guaranteed to be passive.

   All active calls on the same flag form a single total order consisting of zero or
   more exceptional calls, followed by one returning call. The end of each active call
   synchronizes-with the next active call in that order.

   The return from the returning call synchronizes-with the returns from all passive
   calls on the same flag: this means that all concurrent calls to std::call_once are
   guaranteed to observe any side-effects made by the active call, with no additional
   synchronization.

.SH Parameters

   flag    - an object, for which exactly one function gets executed
   f       - Callable object to invoke
   args... - arguments to pass to the function

.SH Return value

   \fI(none)\fP

.SH Exceptions

     * std::system_error if any condition prevents calls to std::call_once from
       executing as specified.
     * Any exception thrown by f.

.SH Notes

   If concurrent calls to std::call_once pass different functions f, it is unspecified
   which f will be called. The selected function runs in the same thread as the
   std::call_once invocation it was passed to.

   Initialization of function-local statics is guaranteed to occur only once even when
   called from multiple threads, and may be more efficient than the equivalent code
   using std::call_once.

   The POSIX equivalent of this function is pthread_once.

.SH Example


// Run this code

 #include <iostream>
 #include <mutex>
 #include <thread>

 std::once_flag flag1, flag2;

 void simple_do_once()
 {
     std::call_once(flag1, [](){ std::cout << "Simple example: called once\\n"; });
 }

 void may_throw_function(bool do_throw)
 {
     if (do_throw)
     {
         std::cout << "Throw: call_once will retry\\n"; // this may appear more than once
         throw std::exception();
     }
     std::cout << "Did not throw, call_once will not attempt again\\n"; // guaranteed once
 }

 void do_once(bool do_throw)
 {
     try
     {
         std::call_once(flag2, may_throw_function, do_throw);
     }
     catch (...) {}
 }

 int main()
 {
     std::thread st1(simple_do_once);
     std::thread st2(simple_do_once);
     std::thread st3(simple_do_once);
     std::thread st4(simple_do_once);
     st1.join();
     st2.join();
     st3.join();
     st4.join();

     std::thread t1(do_once, true);
     std::thread t2(do_once, true);
     std::thread t3(do_once, false);
     std::thread t4(do_once, true);
     t1.join();
     t2.join();
     t3.join();
     t4.join();
 }

.SH Possible output:

 Simple example: called once
 Throw: call_once will retry
 Throw: call_once will retry
 Throw: call_once will retry
 Did not throw, call_once will not attempt again

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to            Behavior as published              Correct behavior
                       std::invalid_argument would be thrown if f
   LWG 2080 C++11      is invalid,                                 removed this error
                       but the scenario where f is invalidated is  condition
                       not specified
   LWG 2442 C++11      the arguments were copied and/or moved      no copying/moving is
                       before invocation                           performed

.SH See also

   once_flag helper object to ensure that call_once invokes the function only once
   \fI(C++11)\fP   \fI(class)\fP
   C documentation for
   call_once
